Body mass index (BMI) is calculated on the basis of your weight and height and is often used as an indicator of obesity. But, says Hemani, researchers don't agree what shapes BMI.

"There's disagreement between the heritability of BMI depending on which study design you use," he says.

Twin studies claim more than 80 per cent of BMI is due to genetic factors while studies involving non-twin family relatives, have found that that the figure is only in the range of 30-50 per cent.

'More robust angle

Hemani says he and colleagues decided to look at the role of genes on BMI "from a more robust angle".

They obtained data on more than 20,000 pairs of siblings and calculated exactly how genetically similar (or dissimilar) each pair was by counting the number of DNA variants each shared across the whole genome.

"We can calculate exactly how much two siblings are sharing directly," says Hemani, adding that this removes the need to make assumptions about the influence on environment on BMI.

Although, on average, siblings were 50 per cent genetically identical, the process Hemani and colleagues went through showed that, in reality, the figure varied from 35-65 per cent, due to random shuffling of the DNA during meiosis

The researchers then looked at how genetic similarity related to BMI and found that it could only account for 42 per cent of BMI variation within pairs.

While the study findings suggest genetics may be less useful in predicting BMI than previously thought, Hemani says 42 per cent is still an important contribution.

In addition, he says, heritability is a "transient concept" because, if the influence of an environmental variable decreases (e.g. diet), this would increase the influence of genes.

Hemani says a large number of genes are involved in influencing BMI and mapping all the genes involved will be important for maximising the predictive power of genetics.

"It's not just a few mutations across the genome that influences high or low BMI but it's the combined effect of probably thousands of mutations across the genome."